Quick drop valves are commonly used in hydraulic systems for bulldozer blades and the like in which the blade is allowed to freefall to the ground under the force of gravity. The fluid expelled from the contracting lift sides of the double acting hydraulic lift cylinders is diverted by the quick drop valves to the expanding drop sides of the lift cylinders to supplement the pump flow thereto. Since the drop sides of the lift cylinders are essentially filled with fluid when the blade comes to rest on the ground, downward force can be quickly applied to the blade for penetrating the ground.
One known quick drop valve control disclosed in U.S. Pat. No. 5,226,348 uses a solenoid valve to direct load generated pressure from the lift side of the lift cylinders for moving a quick drop valve to its quick drop position when a directional control valve reaches a quick drop position so that fluid expelled from the lift sides is directed to the drop sides immediately after the blade begins to freefall. U.S. Pat. No. 5,251,705 discloses a similar type of quick drop valve control.
Bulldozer blades are sometimes used for cleanup operations wherein the blade is allowed to float along the surface and follow the surface contour without operator intervention. Typically, this is done by providing the directional control valve with a float position in which the lift and drop sides of the lift cylinders are interconnected with each other and with both the pump and the tank. Incorporating a float position in the directional control valve increases the length of both the valve body and the control spool and also the complexity of the porting in the valve body. One of the problems encountered with providing a float position in a bulldozer blade lift control system having an integrated quick drop valve is that the float position incorporated in the directional control valve is essentially a duplication of the quick drop position of the quick drop valve. Since both the directional control valve and the quick drop valve by necessity must be sized to handle larger fluid flows during the quick drop operations, providing the float position in the directional control valve unduly increases the cost of the lift control system.
Thus, it would be desirable to provide a quick drop valve control which controls the quick drop valve so that it can be used for both the quick drop and float functions.
The present invention is directed to overcoming one or more of the problems set forth above.
Disclosure of the Invention
In one aspect of the present invention, a quick drop valve control is used with a hydraulic system having a hydraulic pump, a tank, a hydraulic lift cylinder having a drop side and a lift side, and a control valve connected to the pump and the tank and having first and second motor ports connected to the drop and lift sides respectively. The control valve is movable from a neutral position through an intermediate operating position to a fully open position. The quick drop valve control comprises a quick drop valve hydraulically disposed between the control valve and the drop and lift sides of the lift cylinder and has a first position for communicating the first motor port with the drop side and the second motor port with the lift side and a second position for communicating both the drop and lift sides with the first motor port. The quick drop valve has first and second ends and a spring disposed at the first end resiliently biasing the quick drop valve to its first position. A resolver is connected to the drop and lift sides of the lift cylinder and has a resolved pressure port. A valve device has a first position for communicating the first end of the quick drop valve with the lift side of the lift cylinder and for communicating the second end of the quick drop valve with the tank. The valve device also has a quick drop position for communicating the first end of the quick drop valve with the drop side of the lift cylinder and for communicating the second end of the quick drop valve with the resolved pressure port of the resolver. A device moves the valve device to the second position when the control valve reaches the intermediate position.